Ab initio study of reactions of hydroxyl radicals with chloro‐ and fluoro‐substituted methanes

Ab initio calculations at the unrestricted Hartree–Fock (UHF) level have been performed to investigate the hydrogen abstraction reactions of ^? OH radicals with methane and nine halogen‐substituted methanes (F, Cl). Geometry optimization and vibrational frequency calculations have been performed on all reactants, adducts, products, and transition states at the UHF/6‐31G* level. Single‐point energy calculations at the MP2/6‐31++G* level using the UHF/6‐31G* optimized geometries have also been carried out on all species. Pre‐ and postreaction adducts have been detected on the UHF/6‐31G* potential energy surfaces of the studied reactions. Energy barriers, ΔE^?, reaction energies, ΔE_r, reaction enthalpies, ΔH_r, and activation energies, E_a, have been determined for all reactions and corrected for zero‐point energy effects. Both E_a and ΔH_r come into reasonable agreement with the experiment when correlation energy is taken into account and when more polarized and diffuse basis sets are used. The E_a values, estimated at the PMP2/6‐31++G* level, are found to be in good agreement with the experimental ones and correctly reproduce the experimentally observed trends in fluorine and chlorine substitution effects. A linear correlation between E_a and ΔH_r is obtained, suggesting the presence of an Evans–Polanyi type of relationship. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 426–440, 2001